Continuous construction of gradient modulus interphase in CF/PA6 composites with enhanced interfacial properties and reduced porosity

Chopped ultra-thin carbon fiber tape reinforced polyamide 6 (PA6) composites are considered promising materials for balancing the mechanical properties and ease of processing, particularly due to their in-plane quasi-isotropy, which facilitate structural design and manufacturing in the industry. However, further advancement is hindered by the weak interfacial bonding and modulus mismatch between carbon fiber (CF) and PA6 matrix, as well as high porosity of the CF/PA6 composites. In this work, plasma treatment and mixed COOH-carbon nanotubes (CNTs)/PA6 sizing methods are proposed to enhance the surface roughness (Ra) and surface energy of the CF. Compared to untreated CF, the Ra value and surface energy of the modified CF increased by 45.1 % and 69.7 %, respectively. After 0.6 wt% COOH-CNTs modification, the tensile strength, Young's modulus, and interlaminar shear strength (ILSS) of the composites reach 900.0 MPa, 48.4 GPa, and 62.3 MPa, which are respectively 24.9 %, 19.8 %, and 36.9 % higher than those of the unmodified composites. In particular, the porosity is reduced to 1.22 %, which is 73.3 % lower than that of unmodified composites. Moreover, the [email protected]  wt% CNT/PA6 composites exhibit mitigatory modulus gradient across the interphase. This work synergistically enhances the interface adhesion and reduces the porosity of the CF/PA6 composites via a large-scale continuous modification technology.